1. Field of the Invention
The present invention relates to a control device for a vehicle, and more particularly to failure detection of a clutch provided in hybrid a vehicle.
2. Description of the Related Art
In recent years, a hybrid vehicle has been developed that includes both an internal combustion engine and an electric motor used as power sources. A control mode of the hybrid vehicle mainly includes a series mode and a parallel mode. The series mode is a mode in which power of an internal combustion engine drives a power generator, and electric power obtained by the power generator drives an electric motor coupled to a drive axle, thereby causing a vehicle to travel. The parallel mode is a mode in which power of the internal combustion engine drives the drive axle, electric power obtained by a storage battery drives the electric motor coupled to the drive axle, and any one or both of the power of the internal combustion engine and the power of the electric motor are used depending on a driving state of the vehicle to cause the vehicle to travel.
In such a hybrid vehicle, a clutch provided between drive wheels and the internal combustion engine is connected/disconnected to switch between the series mode and the parallel mode.
However, in the hybrid vehicle including the clutch, if closed sticking of the clutch occurs that is sticking of the clutch in a connected state, a rotational speed of the internal combustion engine decreases with decreasing vehicle speed. The decrease in rotational speed of the internal combustion engine provides unstable rotation of the internal combustion engine, leading to poor behavior of the vehicle.
Thus, as a method for determining closed sticking of a clutch, there is a method for determining closed sticking of a clutch based on a gradient of an increase in rotational speed of an electric motor in a released state of the clutch (Japanese Patent Laid-Open No. 2009-202712).
In a drive device for a vehicle described in Japanese Patent Laid-Open No. 2009-202712, a clutch is provided between a transmission and an electric motor, a clutch is provided between the electric motor and an internal combustion engine, and closed sticking of the clutches is determined based on a gradient of an increase in rotational speed of the electric motor in a released state of the clutches.
However, in a hybrid vehicle in which an electric motor and drive wheels are connected without via a clutch, further an internal combustion engine and the drive wheels are connected via a clutch, and the internal combustion engine drives a power generator to generate electric power, the electric motor and the drive wheels are connected without via the clutch. Thus, if a rotational speed of the electric motor is increased to determine closed sticking of the clutch, a vehicle speed of the vehicle also increases, which disadvantageously disturbs behavior of the vehicle.
Also, as another method for determining closed sticking of a clutch, there is a method for determining closed sticking of a clutch from a difference between a rotational speed on an upstream side of power transmission of the clutch and a rotational speed on a downstream side of power transmission of the clutch. However, in a hybrid vehicle in which an electric motor and drive wheels are connected without via a clutch, further an internal combustion engine and the drive wheels are connected via a clutch, and the internal combustion engine drives a power generator to generate electric power, it is likely that a rotational speed on an upstream side of power transmission of the clutch approaches a rotational speed on a downstream side of power transmission of the clutch to reduce a difference between the rotational speeds, thereby causing erroneous determination of closed sticking of the clutch, even in a series mode, depending on a traveling state of the vehicle and a power generation state of the power generator.